fix(finish): gate the global EDA on stem LENGTH, not existence — and report the A/B

The finisher decided whether to run the local-vs-global comparison by testing
whether the global drums stem exists. A soundfile writer creates its file the
instant it opens, so that test is true from the first second of an hour-long pass.
This is the same trap that had me reading "4 of 14 sections done" off a directory
count earlier tonight, and it would have been worse here than a wrong progress
number: the comparison seeks into the global stems by ABSOLUTE TIME, so a
truncated global file does not fail, it silently mis-aligns every later track and
returns plausible correlations for the wrong audio.

Now it probes the duration and requires ~3700 of 3779 seconds before attempting
the global half, falling back to the section-only EDA with the shortfall printed.
In practice the unit is ordered After= the demucs producer so the stems should be
complete anyway — but "should be" is exactly the assumption that a demucs failure
mid-global would have quietly violated, and the finisher runs unattended while PLN
sleeps.

Also made the summary report the air-shelf A/B explicitly, since that pair is the
thing he has to judge: both masters listed with size, sample rate, channels and
duration, and a loud MISSING line if either did not render. The EDA settled that
the 8-16 kHz deficit is cymbals rather than codec noise (drums hold 0.367% of
their own energy up there against 0.084% for the runner-up, a 4.4x concentration),
which licenses opening the shelf but says nothing about how far — so the summary
puts the two candidates side by side rather than announcing a winner.

armada/tide-table/finish_cosmic.sh

@@ -45,15 +45,42 @@ echo "--- split + verify ---"
 "$PY" "$TT/render_release.py" "$SPEC" --only verify
 
 echo
-echo "--- stem EDA (needs the demucs unit to have finished) ---"
-if [ -d "$ROOT/stems_demucs/global" ] && [ -f "$ROOT/stems_demucs/global/drums.wav" ]; then
+echo "--- stem EDA ---"
+# Gate on LENGTH, not existence. A soundfile writer creates the file the instant
+# it opens, so `-f` is true from the first second of an hour-long pass — the same
+# trap that made me misread "4 of 14 sections done" earlier tonight. The global
+# comparison seeks into these by absolute time, so a truncated global stem would
+# silently mis-align every later track rather than fail.
+GDUR=0
+if [ -f "$ROOT/stems_demucs/global/vocals.wav" ]; then
+    GDUR=$(ffprobe -v error -show_entries format=duration -of csv=p=0            "$ROOT/stems_demucs/global/vocals.wav" 2>/dev/null | cut -d. -f1)
+    GDUR=${GDUR:-0}
+fi
+if [ "$GDUR" -ge 3700 ]; then
+    echo "global stems complete (${GDUR}s) — full EDA with local-vs-global"
     "$PY" "$TT/eda_stems.py" "$SPEC"
 else
-    echo "global stems absent — running the section-only EDA"
+    echo "global stems incomplete (${GDUR}s of ~3779) — section-only EDA"
     "$PY" "$TT/eda_stems.py" "$SPEC" --no-global
 fi
 
 echo
+echo "--- the air-shelf A/B, for PLN's ears ---"
+# The EDA proved the 8-16 kHz deficit is cymbals (drums 0.367% vs 0.084%
+# runner-up, 4.4x), which licenses opening the shelf but cannot say how far.
+for pair in "+5:$ROOT/Cosmic26_v1_streaming.flac" "+8:$ROOT/ab_air8/streaming.flac"; do
+    g=${pair%%:*}; f=${pair#*:}
+    if [ -f "$f" ]; then
+        printf 'air %s dB  %s  ' "$g" "$(du -h "$f" | cut -f1)"
+        ffprobe -v error -show_entries stream=sample_rate,channels \
+                -show_entries format=duration -of csv=p=0 "$f" | tr '\n' ' '
+        echo
+    else
+        echo "air $g dB  MISSING ($f)"
+    fi
+done
+
+echo
 echo "--- what is on disk now ---"
 for v in streaming club; do
     d="$ROOT/tracks_v1_$v"